Lesser Noddy (Anous tenuirostris)

Brown noddy and lesser noddy live side-by-side on Bird island, and can be hard to distinguish from one another, but when you look for these two distinct differences it should be a breeze:

Lesser noddy has more grey on head and in particular between the eye and nostrils/beak (the lore).
Brown noddy has distinctly darker, blackish flight feathers (seen over the tail when roosting/perched).

It sounds like this:

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